Recent developments, including those from the applicant laboratory, have opened new opportunities for investigation of dynamic processes on mu s-ms time scales using NMR spin relaxation measurements. Motions on these time scales reflect large-amplitude loop motions, relative motions between domains, collective "breathing" of protein cores, ligand-binding or oligomerization reactions, and overall folding-unfolding events. Such processes may be closely coupled, and in some instances rate-limiting, to biological functions such as molecular recognition, transitions along the catalytic cycle of enzymes, and inhibition or activation of proteins through intra-or inter-molecular protein-protein interactions. Mutations that perturb dynamical processes and conformational equilibria can be associated with significant pathology, including loss or gain of function and misfolding. The existence of large amplitude intra-molecular conformational changes in proteins have been inferred from comparison of equilibrium structures of a given protein in different crystal forms, a given protein in free and ligand-bound states, or sets of homologous proteins. However, only solution NMR spectroscopy can confirm the occurrence and determine the kinetic rates in the solution state of dynamic processes, at equilibrium and with atomic site resolution, in the absence of influences from intermolecular interactions in the solid state, and without potential complications introduced by non-native modifications necessary for other solution-state spectroscopic techniques. The proposed research has four primary objectives: (1) elucidation of the folding mechanism and description of the unfolded-state ensemble for the villin headpiece domain HP67; (2) identification of the mechanistic basis for coupling between agonist or antagonist binding and function of the ionotropic glutamate receptor GluR2 S1S2 ligand-binding domain; (3) assessment of the role of conformational mobility in catalysis by ornithine decarboxylase; and (4) development of novel experimental and theoretical methods for characterizing protein dynamics on mu s-ms time scales. Successful completion of these goals will enable applications to a wide range of protein systems of biological interest. Time-dependent structural changes underlie the normal function of proteins, and misfunction in genetic diseases, cancer, and other pathologies. The proposed research will measure these changes for three model protein systems involved in cellular structure, nerve transmission, and basic metabolism.